Method for preparation of alpha sources of polonium using sulfide micro-precipitation

ABSTRACT

A method for preparing alpha sources of polonium. A sample of polonium is provided in a solution. A controlled amount of sulfide and a controlled amount of a metal capable of forming an insoluble sulfide salt in the solution are introduced into the solution, in order to co-precipitate polonium from the solution. The precipitates are filtered out.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure claims priority from U.S. provisional patent application No. 61/805,626, filed Mar. 27, 2013, the entirety of which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to methods for preparation of alpha sources of polonium. In particular, the present disclosure relates to methods using sulfide micro-precipitation.

BACKGROUND

Polonium-210 (²¹⁰Po) is naturally present at trace levels in the environment as a part of the uranium-238 (²³⁸U) decay chain. It is typically considered as one of the most radiotoxic nuclides: only one microgram of this alpha emitter (t_(1/2)=138 d) may be sufficient to be fatal to an average adult, making it around 250000 times more toxic than hydrogen cyanide^(1,2). Due to its toxicological properties, studies have been done to determine ²¹⁰Po in a variety of samples such as soils, sediments, water, food, tobacco leaves, cigarettes, urine, and biological materials³⁻¹².

Polonium (Po) samples for alpha counting are typically prepared by spontaneous plating on metallic discs. Although silver discs have typically been used for Po plating¹³, nickel, copper, and stainless steel discs may also be employed due to their lower costs^(14,45). Prior to being used, the metallic discs are typically polished and cleaned to remove the dust and the oxide layer at the surface¹⁶. They are then typically brought in contact with the sample in a minimum volume of diluted HCl solution (typically about 0.1 to 1 M) and agitated for about 3-6 hours at a higher temperature (e.g., 80-95° C.) to obtain the highest yields possible (typically about 90%)^(8,13-16.) The metallic discs are typically subsequently rinsed with water¹⁷ and heated at relatively high temperatures (typically about 300° C.) for few minutes to oxidize the polonium and reduce the risk of contamination to the alpha detector¹⁵. Although this sample preparation technique is widely performed, this technique, in particular the heating step, may be inconvenient and time consuming. In addition, the plating is typically performed using in-house assemblies resulting in a low analysis throughput.

SUMMARY

In some example aspects, the present disclosure provides a method for preparing alpha sources of polonium, which may include: providing a sample of polonium in a solution; introducing a controlled amount of sulfide and a controlled amount of a metal capable of forming an insoluble sulfide salt in the solution, in order to co-precipitate polonium from the solution; and filtering out the precipitates.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the drawings, which illustrate example embodiments of the present disclosure, and in which:

FIG. 1 is a chart showing example polonium yields for different amounts of added copper;

FIG. 2 is a chart showing example interferences of polonium yields by different transition metals added at different amounts;

FIG. 3 is a chart showing example interferences of alpha energy resolution for polonium by different transition metals added at different amounts;

FIG. 4 is a chart showing example polonium yields for different reaction times;

FIG. 5 is a chart showing example polonium yields for different HCl concentrations;

FIG. 6 is a chart showing example polonium yields for different sample volumes;

FIG. 7 is a chart showing correlation between expected and measured polonium counts;

FIG. 8 is a table of solubility product constants for some example sulfide salts; and

FIG. 9 is a table of decontamination factors and potential radionuclide interferences for some example polonium isotopes.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

In the presence of sulfide, Po²⁺ is expected to be insoluble in 1 M HCl with a solubility product constant of about 5×10⁻²⁹ (see FIG. 8)¹⁸. This low solubility has been applied to separate polonium sulfide (PoS) from tellurium and bismuth in 1 M HCl¹⁹. As shown in FIG. 8, mercury, silver and copper sulfides are also insoluble sulfide salts²⁰, which may enable their use as co-precipitating agent of PoS for the preparation of thin-layer counting sources by alpha spectrometry. The present disclosure considers the use of certain sulfide salts (such as certain transition metal salts), in particular CuS. However, other sulfide salts may be suitable, such as sulfide salts with low solubility (including those that may not be listed in FIG. 8, such as Cr and Co sulfide salts). For example, sulfide salts of Fe, Pb, Ni, Cd, Co, Cr, Mn, Tl and Zn may be suitable. Investigations similar to those described herein may be carried out to determine the suitability of using such other sulfide salts, as well as the suitable reaction conditions when using such other sulfide salts.

Although the present disclosure describes investigation of reaction conditions for micro-precipitation of polonium from a solution containing HCl, other solutions (e.g., acidic or non-acidic solutions) may be suitable. Although the described example investigations consider HCl being added to provide an acidic solution, other acids (e.g., hydrofluoric acid, phosphoric acid or sulfuric acid) may be used in order to achieve an acidic solution. Investigations similar to those described herein may be carried out to determine the suitability of a given solution, as well as the suitability of other reaction conditions.

Based on the present disclosure, relatively large batches of Po samples may be relatively rapidly processed to increase sample analysis throughput. A vacuum box system may be suitable for such an application of the present disclosure. Micro-precipitation methodologies using lanthanide fluoride for actinides²¹⁻²³ and barium sulfate for radium-226 (²²⁶Ra) have been employed for the preparation of thin-layer counting sources by alpha spectrometry^(24,25), however techniques used in micro-precipitation of actinides and radium-226 typically are not expected to work for micro-precipitation of polonium.

In various examples and embodiments, the present disclosure may provide a relatively robust, simple and/or fast method for the preparation of polonium counting sources for alpha spectrometry using sulfide micro-precipitation, for example using copper sulfide micro-precipitation. Although copper sulfide is discussed herein as an example, other sulfide salts may be suitable. Copper may be practically useful, for example compared to silver and mercury, as silver is typically light sensitive and less stable in solution (and may result in poor spectral resolution in alpha spectrometry); while mercury may be undesirable due to its toxicity. However, use of silver sulfide and/or mercury sulfide for co-precipitation of polonium may be appropriate in some applications, and is within the scope of the present disclosure.

The present disclosure discusses suitable conditions for the co-precipitation of PoS, and examples of suitable ranges are described. Other ranges and sub-ranges may be possible.

In various example studies, potential radionuclide and chemical interferences were also investigated. The possibility of using ²⁰⁹Po as a yield tracer to determine ²¹⁰Po was also investigated.

In the disclosed examples, described further below, Po was co-precipitated with CuS, filtered onto Eichrom Resolve™ filters and counted. The disclosed method may be faster, cheaper, and/or more convenient than conventional spontaneous plating on metallic discs and example studies disclosed herein found that similar yields may be obtained (about 80-90%).

In example studies, described below, suitable conditions for the micro-precipitation method using CuS as co-precipitate were found (e.g., about 50 μg of Cu²⁺ in about 10 mL of about 1 M HCl). These reaction conditions may be compatible with conventional preparation and purification procedures for polonium samples (typically using about 0.1 to 1 M HCl). The example results showed that most susceptible radionuclide interferences (e.g., Ra, Th, U, Np, Pu and Am) for polonium isotopes (namely, ²⁰⁸Po, ²⁰⁹Po and ²¹⁰Po) may be effectively removed. The effects of several transition metals (namely, Cu²⁺, Ag⁺, Fe³⁺, Fe²⁺, Pb²⁺ and Ni²⁺) on the yield and the resolution of alpha peaks were also assessed. The example results demonstrated the versatility of the presently disclosed method for environmental and/or biological matrix. In various example studies, the disclosed method has been applied to determine various amounts of ²¹⁰Po using ²⁰⁹Po as a yield tracer.

Development of an example method for micro-precipitation of Po using a sulfide salt as co-precipitate is now described. This example is provided for the purpose of illustration only and is not intended to be limiting. For example, although CuS is described as an example co-precipitate, other sulfide salts may be used, and may be appropriately selected (e.g., based on solubility product constants). Similarly, although HCl is described as being added to achieve an acidic solution, other acids may be added, or the solution need not be acidic. Certain example reaction conditions are also described as being suitable. These are also provided for illustration only and may be varied as appropriate, for example using appropriate investigation to determine suitability.

Suitable Reaction Conditions

Example studies were carried out to determine suitable reaction conditions for obtaining polonium using micro-precipitation, with CuS as co-precipitate. Conditions that were investigated included: amount of Cu²⁺ added to the solution, reaction time before filtering out precipitates, concentration of HCl in the solution, and total volume of solution used in the reaction. In these investigations, temperature and pressure were kept at ambient levels, as this may be more practical to implement. The disclosed investigations also arrived at a set of reaction conditions that were found to be particularly useful. However, other reaction conditions may be suitable. Similar investigations may be readily carried out to determine suitable reaction conditions using other sulfide salts as co-precipitate.

For the solutions investigated, about 50 mBq of ²⁰⁹Po was added in disposable 50 mL conical polypropylene tubes. Suitable amounts of Cu²⁺ for the reaction was first investigated by adding known quantities of Cu²⁺ from a copper solution (about 500 mg/L in 1% v/v HCl). The co-precipitation was carried out in about 10 mL of about 1 M HCl by adding about 1 mL of 0.3% m/v Na₂S solution. Then, the influence of the precipitation time was investigated. Using selected suitable conditions (in these examples, about 50 μg of Cu²⁺ and a reaction time of about 10 minutes), the influence of the HCl molarity and volume of the solution was investigated.

FIG. 1 shows example results of micro-precipitation yields of ²⁰⁹Po as a function of Cu²⁺ amount added in about 10 mL of about 1 M HCl. Without the addition of Cu²⁺, a yield of 25±2% was obtained. By adding controlled amounts of Cu²⁺, the yield was found to improve. As shown in the example results, with the addition of about 1 μg of Cu²⁺, a yield of around 80% was reached. The yield was substantially the same when controlled amounts of about 5 μg, about 25 μg, about 50 μg and about 100 μg of Cu²⁺ were added. The yield obtained was close to the conventional spontaneous plating technique (typically about 90% in optimal conditions).

The low yield observed when Cu²⁺ was not added suggests that part of divalent or tetravalent Po was precipitated as PoS, which may be consistent with its low solubility (see FIG. 8). This may also be consistent with the observed improved yield when a co-precipitating agent (in this case, Cu²⁺) is added. Note that Po⁴⁺ is a relatively strong oxidant (E°=1.03 V) compared to S²⁻ (E°=0.14 V)²⁷ and is expected to be reduced to Po²⁺ in those conditions, suggesting that no valence adjustment is required.

Polonium yield was also investigated as a function of different interfering transition metals added, at different amounts (see FIG. 2). The spectral quality of alpha spectrometry of the resulting alpha counting source was also investigated (see FIG. 3).

From these example results, it was determined that a controlled amount of as little as about 1 μg of Cu²⁺ added to about 10 mL solution of polonium in HCl may be sufficient to obtain an acceptably high yield of polonium. It may be useful to introduce more Cu²⁺, in order to ensure that a sufficiently high yield is obtained, and to insure against the possibility that Cu²⁺ is caught up by impurities.

However, as shown in FIGS. 2 and 3, using too large an amount of Cu²⁺ (e.g., much more than about 100 μg in a 10 mL solution) may be undesirable, as the co-precipitate obtained may have unacceptably low polonium yield (see FIG. 2) and/or may have poor energy resolution for alpha spectrometry (see FIG. 3). This drop in performance may be due to self-absorption of alpha particles in the thicker counting sources.

In the example investigations, about 50 μg of Cu²⁺ added to about 10 mL of solution was found to be suitable. A lower quantity of added Cu²⁺ may also be suitable, for example depending on the specific sample matrix.

Using more than a minimum amount of Cu²⁺ may also have a practical merit, since the formation of the brown colloidal CuS precipitate in the solution and on the filters may be observed, which may be convenient for routine laboratory work.

FIG. 4 shows Po yield as a function of time. These example results were obtained using about 50 μg of Cu²⁺ in about 10 mL of about 1 M HCl, and measuring Po yield after a reaction time of about 10 min, about 20 min, about 30 min, about 1 hr, about 2 hr, about 3 hr and about 4 hr.

The example results show that a Po yield around 80% was achieved after about 10 minutes. This yield was substantially the same up to at least 3 hours and slowly decreased afterwards. It was observed that, beyond 3 hours, the brown colloidal precipitate slowly coagulated with time and adhered onto the surface of the plastic tubes. After about 24 hours, the precipitate was completely adsorbed on the surface of the tubes leading to a clear solution. Thus, filtration may be conducted within about 3 hours after the addition of the sulfide, in order to avoid the loss of the precipitate due to adsorption.

These example results also indicate that, using the disclosed method, a sufficient polonium yield may be obtained in as little as about 10 min, which may be advantageously faster than the conventional spontaneous plating methods, which typically takes at least 3 hours to reach an equivalent yield.

FIG. 5 shows example results from an investigation of the influence of HCl molarity on the Po yield. This example study was carried out using about 10 mL of the solution, about 50 μg of Cu²⁺ over a reaction time of about 10 minutes. For relatively low HCl molarities (e.g., about 0.01 to about 1 M), the yield remained about the same (at about 80%), but decreased for higher HCl concentrations (e.g., above about 1 M). Solutions with concentrations higher than about 1 M were almost colorless and little precipitate was formed. The precipitates found on the filters obtained from reactions using about 0.01 M solutions were darker and less granular than those from reactions using about 1 M HCl. The FWHM (full width half maximum) of the alpha peak for ²⁰⁹Po was wider for the precipitates obtained using about 0.01 M HCl (150 keV) in comparison to those obtained using about 0.1 M HCl (55 keV) and about 1 M HCl (32 keV) (example results not shown).

At lower HCl molarities, the crystallinity of the precipitates might be different, resulting in the observed variation in color and poor alpha energy resolution. For higher HCl molarities (e.g., above about 1 M), the loss in Po yield may be due to PoS and CuS salts being more soluble at lower pH and practically not precipitated and/or PoS and CuS not being precipitated because H₂S was formed too fast and immediately vaporized, which may have prevented the micro-precipitation from occurring.

To better understand the observed behavior, tests were carried out. To a solution of about 3 M HCl, 7 times more sulfide than normal was added, which formed the brown CuS precipitate with a yield of 93±4%. For a higher concentration of HCl (about 10 M), no precipitate was observed even though excessive amount of solid sodium sulfide was added to the solution and a significant amount of gaseous H₂S was produced. Another test was performed to first form a CuS precipitate in about 1 mL of about 1 M HCl; about 20 mL of concentrated HCl was then added, which brought the concentration to approximately 10 M; but the brown CuS precipitate remained undissolved with a yield of 77±4% for ²⁰⁹Po. The results of these tests suggested that the low Po yield in low pH conditions likely was not caused by the dissolution of the precipitate, but rather by the fast vaporization of H₂S that made the precipitate more difficult to form.

Based on the results of these example studies, it was determined that a solution using a HCl concentration between about 0.1 and about 1 M may be suitable for co-precipitation of Po and CuS. Higher concentrations of HCl may help to reduce some potential interferences, thus HCl at a concentration of about 1 M HCl may be more useful.

FIG. 6 shows example results from an investigation of the effect of solution volume on the micro-precipitation yield. The example investigation was carried out using about 1 M HCl and about 50 μg of Cu²⁺ in solution volumes of about 5 mL, about 10 mL, about 20 mL, about 30 mL, about 40 mL and about 50 mL.

The Po yields were found to be 83±3% for reactions carried out using solution volumes of about 10 mL or less, and slightly decreased for reactions carried out using solution volumes of about 20 mL (75±3%). However, for reactions carried out using solution volumes of about 30 mL or more, the Po yields dropped to almost zero and the solutions became colorless. It was found that it was possible to achieve an acceptable yield (90±4%) by adding 7 times more sulfide to about 40 mL of about 1 M HCl solution. Since a larger amount of HCl may facilitate the formation of H₂S and prevent the precipitation, more sodium sulfide may need to be added to maintain a sufficiently high concentration of S²⁻ in the solution to initiate the micro-precipitation.

As described above, Po micro-precipitation with CuS as a co-precipitate may be achieved relatively quickly (e.g., in about 10 min, and up to about 3 hr) with sufficiently high yields (e.g., about 80% or greater) in about 5 to about 10 mL of about 0.01 to about 1 M HCl using about 1 to about 100 μg of Cu²⁺. In particular, a sufficiently high yield of Po was found to be obtained with CuS as a co-precipitate, using a reaction time of about 10 min, a solution volume of about 10 mL, HCl concentration of about 1 M and about 50 μg of Cu²⁺.

An example suitable method for co-precipitation of polonium with CuS is described below. Suitable conditions for this example method were determined based on the investigations described above. Variations on this example method may be possible.

In 50 mL polypropylene conical tubes, about 50 mBq of ²⁰⁹Po may be added and mixed into about 10 mL of 1 M HCl. For each sample, 7.87×10⁻⁷ moles of Cu²⁺ (about 50 μg) followed by 4.17×10⁻⁵ moles of S²⁻ may be added and the sample vigorously shaken. After sitting for about 10 minutes, the sample may be filtered using a suitable filter, for example through a 0.1 μm Resolve™ filter (Eichrom Technologies Inc., Lisle, Ill.). Prior to the filtration, the hydrophobic filter should be wetted, such as with 1-2 mL of 80% ethanol, followed by 1-2 mL of UPW. The sample may be then filtered, for example at a low flow (approximately 3-4 mL/min) using a vacuum box (Eichrom Technologies Inc., Lisle, Ill.). After the final rinse (e.g., using 1-2 mL of 80% ethanol), the precipitate is dried (e.g., air dried for few minutes) and subsequently mounted on a stainless steel disc, for example using double-sided adhesive tape, for counting by alpha spectrometry.

The present disclosure may provide a useful alternative to the conventional spontaneous plating methodology for the preparation of Po alpha counting sources. The disclosed micro-precipitation method may be faster and easier to operate. It has been found that, for example, using a 12-holes vacuum box for filtration, it may be possible to perform all the preparation steps and process 12 samples within about one hour. Since current conventional spontaneous plating methods reported for the determination of Po are typically performed in a relatively small volume of 0.1 to 1 M HCl, similar to the reaction conditions of the present disclosure, the disclosed micro-precipitation technique may be readily implemented into current practice.

Interference Assessment

Further example studies were carried out to investigate the possibility of interference by radionuclide and other chemicals in examples of the disclosed method.

Radionuclide Interferences

Other alpha emitters, including Ra and actinide nuclides (e.g., Th, U, Np, Pu and Am), may interfere with counting of Po isotopes of interest (²⁰⁸Po, ²⁰⁹Po or ²¹⁰Po) if they were to co-precipitate with the sulfide salt.

Decontamination factors for Ra and example actinides were determined and are shown in FIG. 9, along with their alpha energies and emission intensities of the most susceptible interfering isotopes. An investigation of this possible interference is described below.

For each sample, approximately 50 mBq of Ra and actinide standards were added in about 10 mL of about 1 M HCl. The example CuS micro-precipitation procedure described above (using about 50 μg of Cu²⁺ in about 10 mL of about 1 M HCl, over a reaction time of about 10 min) was followed and the filtrate solution was collected. Radium was determined in the filtrate using a barium sulfate micro-precipitation procedure as previously published by Maxwell²⁵. Actinides were measured in the filtrate using cerium fluoride (CeF₃) micro-precipitation as previously described by Dai^(24,26). The CuS filter and the filtrate samples were both counted by alpha spectrometry, and the decontamination factor was calculated as the ratio of the Ra or actinide activity in the filtrate to that on CuS filter.

A moderate decontamination factor (134) was obtained for Ra; whereas higher decontamination factors (greater than 400) were obtained for actinides. These example results demonstrate that Ra and actinides are not expected to form insoluble sulfides in acidic solutions. Therefore, similar to the conventional spontaneous plating technique, no purification may be required to remove potential radionuclide interferences for Po samples obtained using the disclosed method.

Chemical Interferences

The effects of some example transition metals (in this example study, Ag⁺, Cu²⁺, Fe³⁺, Fe²⁺, Pb²⁺ and Ni²⁺ were considered) that could co-precipitate with sulfide were also evaluated. After the addition of known quantities of those elements and 50 mBq of ²⁰⁹Po, the example CuS micro-precipitation procedure described above (using about 50 μg of Cu²⁺ in about 10 mL of about 1 M HCl, over a reaction time of about 10 min) was applied to prepare the counting sources.

FIG. 2 shows example results illustrating the influence of the example transition metals on the Po yield. FIG. 3 shows example results illustrating the influence of the example transition metals on alpha energy resolution.

For Cu²⁺, which is also the co-precipitating agent, the Po yield remained substantially constant (about 80%) up to about 1000 μg of added Cu²⁺ and then decreased to 25±3% for about 10000 μg of added Cu²⁺ (see FIG. 2). The FWHM increased as the amount of added Cu²⁺ increased. For the Po samples obtained from solutions containing more than 100 μg of added Cu²⁺, FWHM of about 328 keV was reached with about 10000 μg of added Cu²⁺ (see FIG. 3). The precipitates on the filters were observed to be darker as the amount of added Cu²⁺ increased.

Similar results were obtained for Ag⁺. The Po yield was relatively stable up to about 1000 μg of Ag⁺ added to the solution and decreased afterwards (see FIG. 2). The energy resolution was more affected for the Po samples obtained from solutions with more than 100 μg of added Ag⁺ (see FIG. 3), and the filters were observed to be darker as the amount of added Ag⁺ increased. Because of the relatively low solubility of silver sulfide (see FIG. 8), silver was expected to completely precipitate in the presence of S²⁻, resulting in a lower yield and poor energy resolution due to the self-absorption of alpha particles by the thicker precipitate.

Although not tested, it is expected that Hg²⁺ would behave similarly to Ag⁺ due to its low solubility (see FIG. 8). For biological and environmental samples with a high concentration of Cu²⁺ or Ag⁺, additional purification (e.g., using an Eichrom Sr Resin)²⁸ may be required to reduce these chemical interferences before the micro-precipitation.

The example results also show that the micro-precipitation Po yield decreased as the amount of Fe³⁺ added to the solution surpassed about 100 μg and a minimal yield was found at about 1000 μg of added Fe³⁺ (40±3%). The Po yield increased to 62±3% at about 10000 μg of added Fe³⁺ (see FIG. 2). However, the alpha energy resolution was found to be not affected as the amount of added Fe³⁺ increased (see FIG. 3). The brown color characteristic of CuS was observed on the filters for the precipitate obtained from solutions containing about 100 μg of added Fe³⁺ or less; but the filters for the precipitate obtained from solutions containing about 500 and about 1000 μg of added Fe³⁺ were white. For the precipitate obtained from solutions containing about 10000 μg of added Fe³⁺, the filter was pale yellow. Also, the CuS precipitate formed very slowly for the solutions with about 100 μg of added Fe³⁺ and no visible coloration in the solutions was observed for higher added Fe³⁺ quantities.

These results suggest that the precipitation of CuS was hampered as excessive ferric ion might compete with Cu²⁺ and form a complex with S²⁻ in the solution. It may be that, as the ferric sulfide complex was quickly formed, fast consumption of the S²⁻ in the solution and high solubility of ferric sulfide prevented the micro-precipitation of CuS from occurring.

To verify this hypothesis, the solubility of ferric sulfide was examined. In about 1 M HCl solution containing about 1000 μg of Fe³⁺, 10 times more sulfide was added, which changed the yellow complex of FeCl₂ ⁺ to colorless with no precipitate formed. Furthermore, the black Fe₂S₃ precipitate prepared in water was found to be soluble in about 1 M HCl and the H₂S gas was produced. These tests indicate solubility of Fe₂S₃ in 1 M HCl. In another test, 4 times more sulfide was added to a solution of about 1000 μg of added Fe³⁺, and an improved yield of 93±4% was achieved. For the solution containing about 10000 μg of added Fe³⁺, a pale yellow precipitate was observed, possibly due to the formation of trace Fe₂S₃ that adsorbed FeCl²⁻ in the presence of high concentration of Fe(III) in the solution. For verification, a test was performed by filtering a mixture of S²⁻ and about 10000 μg of added Fe³⁺ with no Cu²⁺ added, and a yellow precipitate was observed. In addition, the alpha energy resolution was found to be not affected by the amount of Fe³⁺ added, confirming that only low quantity of the precipitate was produced.

For Fe²⁺, the Po yield was found to be consistently high except for about 10000 μg of added Fe²⁺ (46±3%, see FIG. 2). The alpha energy resolution was found to be not affected (see FIG. 3). The filters showed the characteristic brown color of CuS precipitate, except that the precipitates obtained from solutions with about 10000 μg of added Fe²⁺ were white. A slower CuS precipitation was observed for the solutions of about 1000 μg added Fe²⁺. Adding more sulfide to a solution of about 10000 μg added Fe²⁺ increased the yield to 87±4%. Since ferrous ion is more soluble in 1 M HCl with sulfide than ferric ion (see FIG. 8), a higher quantity of ferrous iron (about 10000 μg) may be needed to compete for S²⁻ and interfere with the CuS precipitation.

These results may be useful since Fe(OH)₃ pre-concentration procedures are typically used for the determination of Po in environmental and biological samples^(13,16). A reduction of Fe³⁺ to Fe²⁺ may be helpful to alleviate the influence of Fe³⁺ on the recovery.

For Pb²⁺, the Po yields were relatively constant at 85±5% for the different amounts of added Pb²⁺ (see FIG. 2), but the FWHM value was found to increase considerably for the solution of about 10000 μg added Pb²⁺ (about 308 keV) (see FIG. 3). The filters had the characteristic brown color of CuS except that the precipitate obtained from the solution with about 10000 μg of added Pb²⁺ was dark black. The solubility of PbS is slightly higher than Fe₂S₃ (see FIG. 8), which may lead to less interference of Pb²⁺ on the micro-precipitation than Fe³⁺.

For solutions with added Ni²⁺, the Po yield and the FWHM were not affected in the quantity range studied (see FIGS. 2 and 3). All the filters had the brown color of CuS precipitate. This may be expected since NiS is expected to be completely soluble in 1 M HCl.

For the solutions with less than about 100 μg of transition metal impurities, no additional purification step may be needed. Similar to the conventional spontaneous plating technique, the interfering transition metals for the CuS micro-precipitation technique may be removed using suitable additional sample pre-treatment steps such as extraction chromatography^(13,28). The addition of such a purification step may be dependent on the sample matrix used.

Determination of ²¹⁰Po in Spike Samples

To evaluate the performance of the disclosed micro-precipitation method, replicate samples spiked with known amounts of ²¹⁰Po were analyzed using ²⁰⁹Po as the tracer for yield correction.

A set of samples spiked with known amounts of ²¹⁰Po were prepared by adding 5-100 mBq of ²¹⁰Pb standard (in secular equilibrium with its daughter ²¹⁰Po) to about 10 mL of about 1 M HCl. Then 50 mBq of ²⁰⁹Po tracer was added to the spike and blank samples for yield monitoring and correction. All the samples were then processed through the micro-precipitation procedure described above (using about 50 μg of Cu²⁺ in about 10 mL of about 1 M HCl, over a reaction time of about 10 min), and the counting sources were prepared for the determination of ²¹⁰Po by alpha spectrometry.

Example results are shown in FIG. 7. As shown, the measured activities of ²¹⁰Po in the spiked samples ranging from about 5 to about 100 mBq agreed with the expected values. Good linearity (slope=1.0141) and correlation (R²=0.9999) were observed, demonstrating acceptable accuracy and precision of the disclosed method for the determination of ²¹⁰Po in environmental and biological samples.

The present disclosure may provide a relatively fast method for the preparation of alpha counting sources of polonium using sulfide micro-precipitation. In particular, CuS co-precipitation was investigated. The disclosed method may be relatively robust, rapid and simple, and may be easier and faster than conventional spontaneous plating methods for the measurement of ²¹⁰Po by alpha spectrometry.

Since the disclosed method may not require the use of a relatively expensive silver disc, the disclosed method may help to reduce the cost of Po analysis.

The disclosed micro-precipitation technique may help to increase the sample analysis throughput and/or reduce the analysis cost. Thus, the disclosed method may be useful for ²¹⁰Po radioassays in emergency samples, among other applications.

In some examples (e.g., using about 0.1 to about 1 M HCl), the disclosed method may be compatible with typical conventional sample preparation procedures for ²¹⁰Po using ion exchange or extraction chromatography purification techniques.

Potential interferences of alpha emitting radionuclides and transition metals on the micro-precipitation yield and alpha energy resolution were also examined in example studies. Example results suggest that the disclosed method may be suitable to be adapted for the determination of Po in a variety of sample matrices by alpha spectrometry. The disclosed method may be applicable to routine and/or emergency radioanalytical procedures for the measurement of Po in environmental and/or biological samples.

The embodiments of the present disclosure described above are intended to be examples only. Alterations, modifications and variations to the disclosure may be made without departing from the intended scope of the present disclosure. While the systems, devices and processes disclosed and shown herein may comprise a specific number of elements/components, the systems, devices and assemblies could be modified to include additional or fewer of such elements/components. For example, while any of the elements/components disclosed may be referenced as being singular, the embodiments disclosed herein could be modified to include a plurality of such elements/components. Selected features from one or more of the above-described embodiments may be combined to create alternative embodiments not explicitly described. All values and sub-ranges within disclosed ranges are also disclosed. The subject matter described herein intends to cover and embrace all suitable changes in technology. All references mentioned are hereby incorporated by reference in their entirety.

REFERENCES

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1. A method for preparing alpha sources of polonium, comprising: providing a sample of polonium in a solution; introducing a controlled amount of sulfide and a controlled amount of a metal capable of forming an insoluble sulfide salt in the solution, in order to co-precipitate polonium from the solution; and filtering out the precipitates.
 2. The method of claim 1, wherein the metal is a transition metal.
 3. The method of claim 2, wherein the transition metal is selected from the group comprising: copper, silver, mercury, iron, lead, nickel, cadmium, cobalt, chromium, manganese, thallium and zinc.
 4. The method of claim 1, wherein the metal is copper.
 5. The method of claim 4, wherein the controlled amount of copper is in the range of about 1 μg to about 100 μg.
 6. The method of claim 5, wherein the controlled amount of copper is in the range of about 30 μg to about 70 μg.
 7. The method of claim 6, wherein the controlled amount of copper is about 50 μg.
 8. The method of any one of claims 1 to 7, wherein there is a time period of at least about 10 min after introducing the sulfide and the metal, before filtering out the precipitates.
 9. The method of claim 8, wherein the time period is no more than about 3 hours.
 10. The method of any one of claims 1 to 9, wherein the polonium is provided in an acidic solution.
 11. The method of claim 10, wherein the solution comprises hydrochloric acid.
 12. The method of claim 11, wherein the hydrochloric acid is in the solution at a concentration in the range of about 0.01 M to about 2 M.
 13. The method of claim 12, wherein the hydrochloric acid has a concentration in the range of about 0.1 M to about 1 M.
 14. The method of claim 13, wherein the hydrochloric acid has a concentration of about 1 M.
 15. The method of any one of claims 1 to 14, wherein the polonium is provided in a controlled amount of solution that is in the range of about 5 mL to about 20 mL.
 16. The method of claim 15, wherein the controlled amount of solution is about 10 mL.
 17. The method of any one of claims 1 to 16, wherein filtering out the precipitates comprises filtering the sample using a vacuum box.
 18. The method of claim 17, wherein multiple alpha sources are prepared from multiple samples of polonium in parallel, using the vacuum box.
 19. The method of any one of claims 1 to 18, further comprising drying the precipitates after filtering and mounting the precipitates on a disc for counting by alpha spectrometry. 